Genetic Basis of Colour Vision

Question 1

Describe the evolution of colour vision?

Answer 1

• When primate vision evolved in mammals, there were at first only 2 opsin genes encoding photopigment proteins in cones
• Later, an allele of 1 of opsin genes mutated, producing a pigment protein that responded to previously unseen λs of light
• Then a region of the allele duplicated & inserted, creating a 3rd opsin gene
• Don’t know whether 2-opsin percept was blue-green or blue-red
• 3rd opsin addition created a multiplicative effect
• If a single opsin gives an animal ability to distinguish 100 shades, addition of 2nd opsin is a times 100 effect
• Such a profound expansion of our visual experience actually required very minor genetic alteration
• Just 3 amino acid substitutions account for the 30 nm difference in peak absorption between modern-day red & green cones in humans
• Each change shifts photopigment’s colour spectrum by 5 nm to 15 nm
• A simple change in 1 nucleotide created colour vision spectrum that we perceive as modern homo sapiens

Question 2

Describe what human colour vision is made up of?

Answer 2

• Human colour vision is based on different λs of light absorbed by 3 cone opsin proteins, which are responsible for spectral tuning of cone cells in retina
• Red & green opsins, whose genes reside on X chromosome, are thought to have evolved from an ancestral cone opsin gene that duplicated itself
• 3 amino acid substitutions in red opsin protein account for spectral tuning of green opsin
• At position 180, swapping serine for alanine produces a 6 nm shift of the absorption spectrum; tyrosine to phenylalanine at position 277 provides a 9 nm shift; & changing a threonine to an alanine at position 285 confers another 15 nm shift in maximum absorption.
• Together these 3 changes produce the 30 nm gap between the maximum absorption of the red & green opsins

Red and green are on the X chromosome – which is the sex-linked chromosome
Blue cone/nanometer is sitting on chromosome 7

Question 3

What are the genetics of opsins?

Answer 3

• Rhodopsin, expressed in rod photoreceptor cells, enables animals to see in dim light
• 3 opsins embedded in cell membranes of cones: blue light absorbing short λ, green light absorbing medium λ, or red light absorbing long λ
• Rhodopsin & the cone opsin genes shared enough sequence homology for known sequence to serve as a probe for unknown genes
• 2 of them—the red & the green—reside on X chromosome and are 96% similar in their amino acid sequence
• Results provided support for idea that an ancient X-linked opsin gene underwent a single duplication event & that subsequent mutations in copy shifted absorbance spectrum of photopigment

Question 4

Describe how opsins work?

Answer 4

• Gene duplication of X-linked opsin was necessary to grant all the animals in a group, some colour vision & this included the males
• There needed to be some mechanism to ensure that both genes on X chromosome were not co-expressed in same cone cell
• Prevailing model for how brain discriminates colours is that it assigns cone classes—green, red, or blue—to each cell by comparing how it & its neighbours respond to various λs
• For instance, if red light hits eye & one cone activates while an adjacent cone stays silent
  o Brain thinks that these are 2 different types of cone
  o However, cone cells are only useful in discriminating colours in this way if each cell expresses only 1 type of opsin (which does not happen)
• If individual cone carried 2 different opsins & responded to absorption spectra of each, its firing wouldn’t be very informative

Question 5

Describe how opsins work on the chromosome and the mechanism of action and what would happen if this was not present?

Answer 5

• Showed in transgenic mice that enhancer sequence is required for expression of red & green opsins & that it selects the one that will be transcribed.
• Opsin gene duplication on X chromosome did not include enhancer, resulting in a single enhancer being responsible for turning on both genes
• It acts on only 1 opsin gene in any given cell—likely chosen at random—meaning that enhancer will lead to expression of red opsin in 1 cone & green opsin in another
• W/o this mechanism, cones would likely express a chaotic & muddled array of green & red opsins, & our perception of colour would be drastically different

Question 6

Describe human opsins, chromosomes, heterozygous, homozygous and the types of colour vision?

Answer 6

• Each cone pigment is encoded by its own gene
• Genes are located on chromosomes. In humans, there are 23 pairs of chromosomes
• One pair of chromosomes is concerned w/ sex determination (referred to sex chromosomes)
• The other 22 pairs are autosomes
• In females the 2 sex chromosomes are XX
  o Females can either be homozygous or heterozygous depending on whether genes on the 2 X chromosomes are same or different, respectively
• In heterozygous state 1 gene is dominant & other recessive
• An individual manifests characteristics of dominant gene
• Males have 1 X chromosome & a dissimilar chromosome labelled Y which does not carry the opsins
• The genes encoding L & M cone opsins are both carried on X chromosome. They are 98% identical to each other (as they evolved from a single gene quite recently in evolutionary time)
• The gene encoding S cone opsin is located on autosome chromosome 7
• During meosis (cell division in embryo) paired chromosomes can exchange parts of their genetic material
• As a result, a chromosome may lose a gene (L or M) or gain a gene (L or M)
  o This may lead to protanopia or deuteranopia
• A hybrid gene may be formed, which combines regions of L & M cone genes into a single gene
  o This may lead to protanomaly (1% of males, 0.01% of females) or deuteranomaly (most common — 6% of males, 0.4% of females)- two types of anomalous trichromacy